The present invention relates to internal and external defibrillation pulses and, more particularly, to exponentially decaying defibrillation pulse waveforms that do not utilize truncation.
Devices for defibrillating the heart have been known for some time now. Implantable defibrillators are well accepted by the medical community as effective tools to combat ventricular fibrillation for an identified segment of the population. A substantial amount of research in fibrillation and the therapy of defibrillation has been done. Much of the most recent research has concentrated on understanding the effects that a defibrillation shock pulse has on fibrillation and the ability to terminate such a condition.
In general, defibrillation shock pulses are delivered through use of a monophasic waveform or, alternatively, a biphasic waveform. A monophasic waveform is typically a single phase, capacitive-discharge, time-truncated, waveform with exponential decay. A biphasic waveform is defined to comprise two monophasic waveforms that are separated by time and that are of opposite polarity. The first phase is designated "PHgr"1 and the second phase is designated "PHgr"2. The delivery of "PHgr"1 is completed before the delivery of "PHgr"2 is begun.
After extensive testing, it has been determined that biphasic waveforms are more efficacious than monophasic waveforms. There is a wide debate regarding the exact reasons for the increased efficacy of biphasic waveforms over that of monophasic waveforms. One hypothesis holds that "PHgr"1 defibrillates the heart and "PHgr"2 performs a stabilizing action that keeps the heart from refibrillating.
Biphasic defibrillation waveforms are now the standard of care in clinical user for defibrillation with implantable cardioverter-defibrillators (ICDs), due to the superior performance demonstrated over that of comparable monophasic waveforms. To better understand these significantly different outcomes, ICD research has developed cardiac cell response models. Waveform design criteria have been derived from these models and have been applied to monophasic and biphasic waveforms to optimize their parameters. These model-based design criteria have produced significant improvements over previously used waveforms.
In a two paper set, Blair developed a model for the optimal design of a monophasic waveform when used for general bodily electrical stimulation. (1) Blair, H. A., xe2x80x9cOn the Intensity-time Relations for Stimulation by Electric Currents.xe2x80x9d I.J. Gen. Physio. 1932; 15:709-729. (2) Blair, H. A., xe2x80x9cOn the Intensity-time Relations for Stimulation by Electric Currents II.xe2x80x9d I.J. Gen. Physiol. 1932; 15:731-755. Blair proposed and demonstrated that the optimal duration of a monophasic waveform is equal to the point in time at which the cell response to the stimulus is maximal. Duplicating Blair""s model, Walcott extended Blair""s analysis to defibrillation, where they obtained supporting experimental results. Walcott , et al., xe2x80x9cChoosing the Optimal Monophasic and Biphasic waveforms for Ventricular Defibrillation.xe2x80x9d J.Cardiovasc. Electrophysio. 1995; 6:737-750.
Independently, Kroll developed a biphasic model for the optimal design of "PHgr"2 for a biphasic defibrillation waveform as applied internally. Kroll, M. W., xe2x80x9cA Minimal Model of the Single Capacitor Biphasic Defibrillation Waveform.xe2x80x9d PACE 1994; 17:1782-1792. Kroll proposed that the "PHgr"2 stabilizing action removed the charge deposited by "PHgr"1 from those cells not stimulated by "PHgr"1. This has come to be known as xe2x80x9ccharge burping.xe2x80x9d Kroll supported his hypothesis with retrospective analysis of studies by Dixon, et al., Tang, et al., and Freese, et al., regarding single capacitor, biphasic waveform studies. See, Dixon, et al., xe2x80x9cImproved Defibrillation Thresholds with Large Contoured Epicardial Electrodes and Biphasic Waveforms.xe2x80x9d Circulation 1987; 76:1176-1184; Tang et al., xe2x80x9cVentricular Defibrillation Using Biphasic Waveforms: The Importance of Phasic Duration.xe2x80x9d J. Am. Coll. Cardio. 1989; 13:207-214; and Freese, S. A. et al., xe2x80x9cStrength Duration and Probability of Success Curves for Defibrillation with Biphasic Waveforms.xe2x80x9d Circulation 1990; 82:2128-2141. Again, the Walcott group retrospectively evaluated their extension of Blair""s model to "PHgr"2 using the Tange and Freese data sets. Their finding further supported Kroll""s hypothesis regarding biphasic defibrillation waveforms as applied to internal defibrillation. For further discussions on the development of the models, reference may be made to PCT publications WO 95/32020 and WO 95/09673 and to U.S. Pat. No. 5,431,686. U.S. Pat. No. 5,431,686 is hereby incorporated by reference.
The xe2x80x9ccharge burpingxe2x80x9d hypothesis may be used to develop equations that describe the time course of a cell""s membrane potential during a biphasic shock pulse. At the end of "PHgr"1, those cells that were not stimulated by "PHgr"1 have a residual charge due to the action of "PHgr"1, on the cell. The xe2x80x9ccharge burpingxe2x80x9d model hypothesizes that an optimal duration for "PHgr"2 is that duration which removes as much of the "PHgr"1 residual charge from the cell as possible. Ideally, these unstimulated cells are set back to xe2x80x9crelative ground.xe2x80x9d The xe2x80x9ccharge burpingxe2x80x9d model proposed by Kroll is based on the circuit model shown in FIG. 1B, which is adapted from the general model of a defibrillator in FIG. 1A. in FIG. 1B, RH represents the resistance of the heart, the pair CM and RM represent membrane series capacitance and resistance of a single cell. C1 represents the "PHgr"1 and "PHgr"2 capacitor set. The node VS represents the voltage between the internal electrodes, while VM denotes the voltage across the cell membrane.
It should be noted that the xe2x80x9ccharge burpingxe2x80x9d model may also account for removing residual cell membrane potential at the end of a "PHgr"1 that is independent of a "PHgr"2 pulse, i.e., "PHgr"2 is delivered by a set of capacitors separate from the set of capacitors used to deliver "PHgr"1. This xe2x80x9ccharge burpingxe2x80x9d model is constructed by adding a second set of capacitors, as illustrated in FIG. 2. In this figure, in addition to those elements described with reference to FIG. 1B, C2 represents the "PHgr"2 capacitor set that is separate from C1.
Contrary to the internal defibrillators/defibrillator circuit models described above, external defibrillators can not deliver electrical shock pulses directly to the heart. Rather, external defibrillators must send electrical pulses to the patient""s heart through electrodes that are applied to the patient""s torso. External defibrillators are useful in any situation where there may be an unanticipated need to provide electrotherapy to a patient on short notice. The advantage of external defibrillators is that they may be used on a patient as needed, then subsequently moved to be used on another patient.
While the moveability of the external defibrillator is indeed a useful advantage, that moveability presents at least two problems not found with internal defibrillators. First, the transthoracic defibrillation problem which results, as explained earlier, from the fact that the external electrodes traditionally deliver their electrotherapeutic pulses to the patients heart by first passing through the patient""s chest. Second, the patient variability problem which results from the fact that external electrodes and defibrillators must be able to be used on patient""s having a variety of physiological differences. To accommodate that variety, external defibrillators have traditionally operated according to pulse amplitude and duration parameters.
The internal defibrillator models described above, do not fully address the transthoracic defibrillation problem or the patient variability problem. In fact, these two limitations to external defibrillators are not fully appreciated by those in the art. For example, prior art disclosures of the use of truncated exponential monophasic or biphasic shock pulses in implantable or internal defibrillators have provided little guidance for the design of an external defibrillator that will successfully defibrillate across a large, heterogeneous population of patients. In particular, an implantable defibrillator and an external defibrillator can deliver a shock pulse of similar form, and yet, the actual implementation of the waveform delivery system is radically different.
In the past five years, new research in ICD therapy has developed and demonstrated defibrillation models and their associated design rules. These models and rules for the development of defibrillation waveforms and their characteristics were first developed by Kroll and Irnich for monophasic waveforms using effective and rheobase concepts. (1) Kroll, M. W., xe2x80x9cA Minimal Model of the Monophasic Defibrillation Pulse.xe2x80x9d PACE 1993; 15:769. (2) Irnich, W., xe2x80x9cOptimal Truncation of Defibrillation Pulses.xe2x80x9d PACE 1995; 18:673. Subsequently, Kroll, Walcott, Cleland, and others developed the passive cardiac cell membrane response model for monophasic and biphasic waveforms, herein called the cell response model. (1) Kroll, M. W., xe2x80x9cA Minimal Model of the Single Capacitor Biphasic Waveform.xe2x80x9d PACE 1994: 17:1782. (2) Walcott, G. P., Walker, R. G., Cates, A. W., Krassowska, W., Smith, W. M., Ideker, R. E., xe2x80x9cChoosing the Optimal Monophasic and Biphasic Waveforms for Ventricular Defibrillation.xe2x80x9d J. Cardiovasc. Electrophysio. 1995; 6:737. (3) Cleland, B. G., xe2x80x9cA Conceptual Basis for Defibrillation Waveforms.xe2x80x9d PACE 1996; 19:1186.
A significant increase in the understanding of waveform design has occurred and substantial improvements have been made by using these newly developed design principles. Block et al., has recently written a comprehensive survey of the new principles-based theories and their impact on optimizing internal defibrillation through improved waveforms. Block, M., Breithardt, G., xe2x80x9cOptimizing Defibrillation through Improved Waveforms.xe2x80x9d PACE 1995; 18:526.
However, there have not been significant developments in external defibrillation waveforms beyond the two basic monophasic waveforms, i.e., the damped sine or the truncated exponential. To date, their design for transthoracic defibrillation has been based almost entirely on empirically derived data with little to no influence by the important developments in ICD research.
Recently, there has been reported research on the development and validation of a biphasic truncated exponential waveform in which it was compared clinically to a damped sine waveform. For additional background, reference may be made to U.S. Pat. Nos. 5,593,427, 5,601,612, and 5,607,454. See also, Cliner, B. E., Lyster, T. E., Dillon, S. M., Bard, G. H., xe2x80x9cTransthoracic Defibrillation of Swine with Monophasic and Biphasic Waveforms.xe2x80x9d Circulation 1995: 92:1634-1643; Bardy, G. H., Gliner, B. E., Kudenchuck, P. J. Poole, J. E., Dolack, C. L. Jones, G. K., Anderson J. Troutman, C., Johnson, G., xe2x80x9cTruncated Biphasic Pulses for Transthoracic Defibrillation.xe2x80x9d Circulation 1995; 91:1768-1774; and Bardy, G. H. et al., xe2x80x9cFor the Transthoracic Investigators: Multicenter Comparison of Truncated Biphasic Shocks and Standard Damped Sine Wave Monophasic Shocks for Transthoracic Ventricular Defibrillation.xe2x80x9d Circulation 1996: 94: 2507-2514. Although research determined a usable biphasic waveform, there was no new theoretical understanding determined for external waveform design. As such, external waveform design is progressing much like that established in the early stages of theoretical ICD research. The limitations of this determined biphasic waveform are likely due, at least in part, to a lack of principles-based design rules to determine waveform characteristics.
Further, the delivery of a biphasic truncated exponential waveform requires additional consideration of the design of circuit components. Generally, a circuit that delivers a truncated exponential waveform uses switching mechanisms or relays to cut off, or truncate, the waveform. These switching mechanisms truncate the waveform during the time that the defibrillation pulse is being delivered and at a time when there is still a great amount of current flowing through the switching mechanism. Because of the great current, the switching mechanisms are subjected to extreme wear and are prone to failure. Attempting to respond to these failures, U.S. Pat. No. 5,748,427 discloses a method and system for detecting switching mechanism failure in external defibrillators. Alternatively, multiple switching devices may be incorporated into a circuit to avoid failure, U.S. Pat. No. 5,405,361 discloses a multiple switching external defibrillator. Whether introducing special design considerations in the form of failure detection or multiple switches, additional time, money and effort are expended.
In view of the above, there is a need in the art for the development of a transthoracic cell response model and design rules which can be used to develop the characteristics of external defibrillation waveforms. Further, there is a need in the art for an optimally designed waveform which requires minimal truncation, specifically a full-tilt exponential waveform which is defined as an exponential waveform truncated at approximately 95% of the full waveform; at 95% truncation approximately 99.75% of the energy of the defibrillation pulse has been delivered. Moreover, there is a need in the art for internal and an external devices which can deliver this waveform.
The needs described above are in large measure met by a full-tilt exponential defibrillation pulse waveform of the present invention. The characteristics of the full-tilt exponential waveform are developed through use of a charge burping model or through use of a transthoracic cell response model. These characteristics are implemented within a pulse delivery circuit, which itself is implemented within an internal defibrillator or within an external defibrillator. The pulse delivery circuit includes at least one switch which starts and stops the delivery of the defibrillation pulse. Delivery of the defibrillation pulse is started by the switch when there is sufficient stored energy to deliver a desired pulse. Delivery of the defibrillation pulse is stopped by the switch when the current flowing through the switch is substantially zero and the exponential waveform of the pulse is substantially fully decayed.